See clip from Attorney Rajiv S. Khanna's conference call video that addresses this question.
When you have this length of delay for two years the best thing to do is first approach your congressman’s office. Go to https://www.congress.gov/ enter your zip code and it tells you who your house of representative members are and who your senator is. Contact either one of them. Send a letter to the representative and tell them you live in their area and you are a constituent and you have a pending naturalization application that has been stuck for two years. If they are not able to resolve it get yourself a lawyer and file for a law suit. Normally I don’t recommend a law suit but when naturalization applications have been delayed so long a law suit is a good bet.